Can cats eat dog food?

My grandpa decided to buy dog food for his cat. Shortly after he began feeding dog food to her, his cat began waking up earlier in the morning,yowling in the hallway, and exhibiting grouchy, uptight, and ornery behavior. He claims that this is just a 'phase' that his cat is going through, but the rest of us are concerned. Is dog food the cause of his cat's unusual behavior? Is the cat getting the nutrients he needs?